The invention relates to a colour display tube comprising first electrode means to generate plurality of electron beams, situated along axes parallel to the main axis of said tube; a display screen on which said electron beams converge; second electrode means situated along the path of the electron beams between the first electrode means and the display screen, which second electrode means form a lens field which focuses the electron beams symmetrically; and third electrode means between the first and the second electrode means with which, if desired in cooperation with the first electrode means, an asymmetric lens field is formed to converge the electron beams on the display screen.
Such a colour display tube is disclosed in U.S. Pat. No. 2,957,106. Such display tubes are used inter alia as tubes to display coloured pictures, as oscilloscope tubes, etc. In such tubes it is desired for the electron beams to be converged in one point on the display screen. In U.S. Pat. No. 2,957,106 an asymmetric electron lens is provided in the path of the electron beams which do not coincide with the main axis of the tube between the triode part of the electron gun formed by the cathode, the first and second grids, and the focusing lens, so that the beams are deflected towards each other and converge on the display screen. The focusing lens is formed by a lens field between two electrodes. These electrodes consist of curved electrode plates having apertures therein. The plates are curved so as to be always perpendicular to the electron path. By applying a potential difference between the plates an electron lens is formed which is symmetrical for the electron beams and which has a focusing effect and focuses each electron beam on the display screen. It is very difficult to manufacture such very accurately curved electrode plates and assemble them with respect to each other. Electrodes of such electron guns are assembled by means of assembly pins which have to enclose a very accurate angle with respect to each other. In order to be able to remove the guns from the assembly pins it is necessary for these pins to be connected detachably in a jig as a result of which their mutual angle becomes less accurate as a result of detrition, diurt, bending an breaking of the pins.
This problem is recognized in U.S. Pat. No. 3,906,279 and a solution to this problem is given. This patent teaches a construction for the convergence of three electron beams from three assembled electron guns whch operate independently of each other and the axes of which are parallel and hence parallel assembly pins can be used. This construction is characterized in that of each electron gun which is situated eccentrically with respect to the main axis of the tube, the last electrode situated on the side of the display screen has an axis which is situated eccentrically with respect to the axis of the relevant electron gun in a plane through the main axis of the tube and the axis of the electron gun and at a larger distance from the main axis of the tube than the axis of the electron gun. This last electrode also has a larger diameter than the other electrodes of the electron gun. As a result of the eccentrically placed last electrodes, convergence of the electron beams is obtained in a simple manner and at the same time the electron beams are each focused separately.
U.S. Pat. No. 3,772,554 discloses an integrated system of electron guns operating in an analogous manner. A system of electron guns operating in an analogous manner and in which the focusing lenses of the guns not situated on the tube axis are asymmetrical is known from German Patent Application 2,406,443 laid open to public inspection. All these constructions are less attractive because they exhibit a very important disadvantage. A variation of the strength of the focusing lens in such guns at the same time has a direct influence on the convergence of the electron beams, which is not desired.